just out of curiosity, how long ago was the wiring done?
Reason being that if the work was done pre (early 2005) there was not system in place at the time regarding Part P.
Although not exact, new cable colours tend to mean the work should have been inside the scope of Part P and so a EIC would have been required.
A periodic inspection report (PIR) will have to be commissioned, but that report may not be accepted by the purchasers or there solicitor as acceptable.
Reason ? Well it sounds as if you have marketed you house as having been fully refurbished and as such that would have meant you had a legal requirement to work inside the rules of part P. Since you haven't got an EIC your work can't be accepted as having been done inside the regulations.
Choices ? Offer a discount on the selling price or get a PIR and agree to pay for any remedial works (if the PIR highlights any issues).
Hubby basically rewired the whole house (although he is not an electrician) he is very competent and was supervised by his brother in law who is a commercial electrician.
Although I'm sure the work is safe, I would have an 'issue' since domestic electricals are a set mind set of training, regulations and design requirements and plenty of commercial guys simply don't have domestic experience.
Such things as 25mm LN tails, 10mm MET, fuse board RCD requirements may not have been considered. If they haven't, which the PIR will report on then it might be best to consider a full CU (fuse board) update to 17th edition July 2008 standard. This will require full site testing and would produce an EIC, cost would vary from £350-£1000 subject to size of CU, number of circuits, amount of work required to correct any defects on initial installation caused by lack of domestic knowledge.